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Quadro cards are much more expensive because they've undergone a much more exacting certification process so as to be used on professional 3D modelling softwares.

Those cards also come with certified drivers, which moreover use less efficient but more precise texture clamping.

You can get a regular Geforce card, soft-mode it to fool the driver into thinking it is a Quadro card, and have basically the same capabilities you'd get with a Quadro, but it would (of course) not be covered by Nvidia's certification and support.

Useful to save quite a few bucks and do personal work, but I wouldn't trust it for professional work - or only as a backup solution.

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What I would like to know is when all these 4 factors
Memory Size , Memory Interface , Graphic Memory Bandwidth , Graphics Bus
are almost same except Bandwidth why the pricing is so high for NVIDIA Quadro FX 3500 AS COMPARED TO NVIDIA Quadro FX1500.

How much performance difference does it make , where exactly it differs?

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Well, the 5 extra shaders represent +20% performance per clock, the +95MHz/+70MHz clock speeds represent a healthy boost... However, since both are using the same core, I wonder why one does stereo while the other does HD-out; it may be a matter of in-box driver (stereo requires dual rendering contexts, HD out merely requires a different output mode)

SLI support requires driver support - thus certification. Software SLI may be possible on the FX 1500 (if there is really no port for the SLI bridge), but not supported anyway - so not certified.

The price difference comes a bit from the upgraded hardware, but (I think) mainly from the difference in driver certification level.